Relation Between Molarity And Molality- Formula, Derivation

Q: Q1: What is Molarity?

Ans: Molarity, also known as molar concentration, is defined as the number of moles of the solute in 1 litre of solution and is denoted by M.

Q: Q2: What is the difference between molarity and molality?

Ans: Molarity (denoted by M) is defined as the number of moles of the solute in 1 litre of solution whereas molality (denoted by m) is the number of moles of solute present in 1 kg of solvent.

Q: Q3: What is the molality of a sodium sulfate solution having 138 gm of sodium sulfate ions in 1 kg of water? a.16 b.8 c.6 d.12

Ans: The complete solution to the question Weight of sodium ions = 138 gm The molecular mass of sodium = 23 gm The total no. of moles of sodium ion = 138/23 = 6 moles Weight of solvent given = 1 kg Hence, Molality = number of moles of sodium ion/ weight of solvent in kg = 6/1 = 6 m (correct option is c)

Q: Q4: Find the molarity of a solution comprising 50 gm of NaCl in 500 gm of a solution having a density of 0.936 gm/cm3

Ans: Density = Mass/volume Volume = Mass/density = 500/0.936 = 534.18 ml of solution Molarity = number of moles of solute/volume of the solution (in litres) =50/58.44 ∗ 1000/534.18 =1.60 M

Q: Q5: Which is more preferred- molarity or molality?

Ans: Molarity is the number of moles of solute present in 1 litre of the solution. Molality is the number of moles of solute per kg of solvent. This indicates that molarity is a measure of the volume of the solution. A change in the temperature of the system is known to affect the molarity of the solution. The molality of a solution is a measure of the mass of the solution which has no effect on a change in temperature. This independence of molality on temperature makes it a preferred choice for expressing solution concentrations.

Q: Q6: Differentiate in terms of concentration: 1M vs 1m?

Ans: 1M indicates 1 mole of solute in 1 litre of solution. We already know that a solution is a mixture of both solute and solvent. 1m indicated 1 mole of solute in 1 kilogram of solution which is 1 litre of solvent. This indicates that the quantity of the solvent is more in 1m of the solution as compared to 1M of the solution (for the same 1 mole of solute). Thus the concentration of 1M of a solution is greater than 1m of the solution having the same number of moles of the solute.

Q: Q7: Define the Normality of a solution.

Ans: Normality is defined as the gram equivalent of solute per litre of solution and is another popular way of calculating the concentration of a solution like the molarity and molality. The S.I. unit of normality (denoted by N) is eq/L. The normality of a solution can be calculated by the formula: N = Weight of Solute in gram / Equivalent Weight × Volume in litre.

Relation Between Molarity And Molality

Archana S,

Mar 4, 2024

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The relation between molarity and molality is expressed by m=1000M / 1000ρ−MM1 where ρ is the density of solution (mg/mL) and M1 is the molecular weight of the solute.

Table of Contents

Relation Between Molarity and Molality
Differences Between Molarity and Molality
Basic Definitions

The relation between molarity and molality is given by m=1000M / 1000ρ−MM₁ where ρ is the density of solution (mg/mL) and M₁ is the molecular weight of the solute. Both molarity and molality are terms used to quantitatively express the strength of the solution (concentrated or diluted). Molarity is the concentration of a solution expressed in terms of moles per unit volume whereas molality is the concentration expressed in terms of moles per unit mass.

Relation Between Molarity and Molality

The relation between molarity and molality is that both play a crucial role in the determination of the concentration of a solution. The derivation of the relation between molarity and molality is given below.

Let us suppose the mass of the given solute is W g (W / 1000kg)

Let the mass of the given solute be ‘W’

Let the volume of the solution be ‘V’

Let the molality be ‘m’

Let the molar mass of solute be ‘M1’

Let the Molarity be ‘M’

Let the weight of the solvent be ‘W1’

Differences Between Molarity and Molality

Both molarity and molality are the terms used to express the composition and concentration of a solution. Find below some of the basic differences between molarity and molality for your reference:

Parameters	Molarity	Molality
Measure of	Concentration of the solution	Concentration of the solution
Defined as	The number of moles of the solute per litre of solution.	The number of moles of solute present in 1 kilogram of solvent.
Represented as	M	m
Equation/formula	Moles of solute/volume of solution (in litres)	Moles of solute/weight of solvent (in kg)
Represented as a ratio of moles to	Volume of solution in litres	Weight of solvent in kgs
Relation Between Molarity and Molality	M=1000d / [(1000 /m)+M₁]

Basic Definitions

Both molarity and Molality are terms used to express the composition and concentration of a solution with respect to the solute and solvent. The basic definitions of various terms associated with molarity and molality are explained below:

Solute, Solvent, and Solution

A solute is the component of a solution present in small quantities.
A solvent is a component of a solution present in large quantities.
The solution is a mixture in a liquid state comprising two components, i.e. a solute and a solvent. Both the solute and solvent mixture forms the solution.

Mole

A mole is defined as a standard scientific unit for measuring large quantities of very small entities such as atoms, molecules, or other specified particles. For example, 12 gms of Carbon-12 atoms represents one mole.

Molarity

Molarity, also known as molar concentration, is defined as the number of moles of the solute per litre of solution. It is denoted using the symbol M (capital m). The formula for molarity is expressed as

Molarity = moles of solute/litres of solution

Molality

Molality is defined as the number of moles of solute present in 1 kilogram of solvent. It is denoted using the symbol m (small m). The formula for molality can be given as

Molality = moles of solute/kilograms of solvent